Transposons have been shown to play major roles in spontaneous mutation in Drosophila and other eukaryotes. In chromosomes, the insertion or excision of these mobile elements disrupts gene function at the site of insertion or creates deletions if excision is not precise. Additionally, recombination between two copies of a transposon situated at different sites in a chromosome or in homologs produce a variety of chromosomal rearrangements depending on the orientation of the paired mobile elements. We are studying rearrangements produced by these types of ectopic recombination to determine the molecular structures at their breakpoint junctions and to learn more about how transposons mediate these types of asymmetrical exchanges. We are also studying an example of high mobility by the retrotransposon gypsy to try to determine what conditions mediate such bursts of amplification and insertion into new chromosomal sites.